I'm sorry, you lost me on that one. I have no idea. I just figured that Nikon's own "flash" setting for white balance and the built in camera flash would be matched or compatible.

The flash emits light, but it is not full spectrum light and it is not the same as the light you were seeing when your eyes saw the purple. In fact, I gel my flashes with a warming filter to balance the light they emit so that my camera's sensors aren't overly deprived of red light and the light coming from the flash is closer to the light we get from the sun. Also, as flashes start to age the spectral properties of the light they emit changes, and different flash units will require different WB adjustments, so we are back to careful measurement of the WB using a calibrated reference (such as a gray card).